You need to parse JSON - date with:
var date = new Date(parseInt(jsonDate.substr(6)));
Source here: https://stackoverflow.com/a/2316066/1278667
EDIT: In your case:
$.post("/BindInventory/GroupName", { tourname: tname, MatchType: MatchType }, function (result) {
var startdate = new Date(parseInt(result.sdate.substr(6)));
var enddate = new Date(parseInt(result.edate.substr(6)));
$('#date').datepicker('option', 'minValue', startdate);
$('#date').datepicker('option', 'maxValue', enddate);
}, "json");